A common form of large industrial or utility electrostatic precipitators comprise an array of vertically extending collecting electrodes which are maintained at a relatively positive polarity and an array of discharge electrodes having points (thorns or barbs) or sharp edges constituting corona-discharge regions. Below the base of the collecting electrodes, suitable hoppers or collecting bins are provided, and the collecting electrodes are associated with rapping means for dislodging the collected dust into the bins. The collecting assembly consists of pairs of such dust-collecting electrodes defining a passage for the gas between them, with arrays of corona-discharge electrodes being positioned intermediate pairs of collecting electrodes.
Generally speaking, the corona-discharge electrodes are at negative polarity and are connected to the opposite terminal of the high-voltage direct-current source. The corona-discharge generated between the edges or barbs (i.e. sharp points) of the discharge electrode and the opposite portion of a collecting electrode results in ionization of gases and the generation of charged particles (i.e. negative ions or electrons), which are picked up by the particulate so that the latter become negatively charged. The particulate is then attracted to and collected upon the positively charged collecting electrodes to which they adhere, electrostatically or mechanically, until the collecting electrode is rapped to dislodge the particulate into the hoppers. The particulate laden gas passes generally horizontally between the vertical collecting electrodes.
With the prior art discussed hereinabove, effectiveness of dust collection is reduced in the case of high-resistance particulate, because of the very well known phenomena of reverse ionization at the side of the collecting electrode at which the particulate accumulates. As a result, positively charged particulate may be released or formed by such reverse ionization and such positively charged particles are repelled from and not attracted to the positively-charged collecting surface. As the gas stream passes between the collecting electrodes, therefore, particles which pick up a positive charge by reverse ionization proximal to a collecting electrode tend to move toward the next discharge electrode at which they may pick up a negative charge and then move toward the collecting electrode where they may again pick up a positive charge, and so on. Viewed as a statistical phenomenon, therefore, particles tend to move in a zig-zag fashion between the plane of the discharge electrodes and the collecting electrodes spaced therefrom as the gas entrains such particles along the collecting path. The zig-zag movement is a phenomenon which is associated with high-resistance particulate, and also exists in situations of very low resistivity particulate, such as carbon dusts.
Because of the zig-zag phenomenon, the effectiveness of particulate collection is obviously reduced and hence the performance of an electrostatic precipitator may be substantially lower for high-resistance particles than with normal resistance particulate. One obvious solution to the problem is to increase the conductivity of the dust which is processed. The art has recognized this and in many cases has provided for the introduction of moisture for the humidification of the dust, or for flue gas treatment with sulfur trioxide and/or ammonia, before the gas stream enters the assembly. Naturally, this procedure cannot be used in all cases and depends upon the nature of the gas stream, the nature of the dust and the parameters under which the system operates. However, it is important to improve the efficiency of dust collecting assemblies for high-resistance dusts.
To some extent the hereinabove mentioned deficiencies of prior generally utilized electrostatic precipitators has been recognized, for example as is illustrated in U.S. Pat. No. 4,326,861.
The '861 Patent adopts a differing construction of the corona-discharge electrodes, whereby the surface of the corona-discharge electrode facing away from the side thereof provided with the corona-generating means, (i.e. edges, points, barbs, or thorns), has a large convex surface area to form a collecting surface for reversely polarized dust resulting from reverse ionization. With such a prior arrangement, the discharge electrodes are so oriented and disposed that the discharging and field-forming parts, alternate with one another in the direction of flow of the gas to be treated. In this regard, by forming the field part of each discharge electrode with a convex contour in the direction of the collecting electrode juxtaposed therewith and of a breadth which significantly exceeds the breadth of the corona-generating means, dust particles at the collecting electrode which have been charged to the polarity at which this electrode is maintained by the reverse ionization process described previously, are attracted to the field-forming part of the discharge electrode and are collected thereby.
More specifically, the dust particles near the arresting or collecting electrode, of the '861 Patent, which have been charged to a plus or positive polarity by the positive ions resulting from reverse ionization, are conveniently collected by the field-forming part of the discharge electrode. Meanwhile, the dust particles around the discharge part (i.e. the region of the corona-generating means) which are charged to negative or minus polarity are caught by the collecting electrode. The foregoing requires, naturally, that the collection electrodes be at a relatively more positive polarity than the discharge electrodes.
Consequently, the zig-zag flow of dust particles attributable to reverse ionization is limited and the performance of the dust-arresting assembly of generally accepted prior art can possibly be improved so that the high-resistance dusts with which reverse ionization is a particular problem, are intercepted with high efficiency.
While the arrangement illustrated in the '861 patent appears to be an advance over the prior art theretofore, a number of deficiencies are apparent with the design illustrated therein. More specifically: non-uniform field voltage is apparent because of the breakdown in the gap between adjacent sections of the discharge electrodes; one cannot practically reverse polarity without losing the majority of the collecting area; the design of the discharge electrodes is relatively expensive and does not necessarily present an arrangement for efficient rapping to dislodge accumulated particulate; and others.